We have characterized the charge-transfer spectra of model imidazole-Cu(II) chromophores and used these results to predict the positions of corresponding absorptions of both planar and pseudotetrahedral Cu(II) protein chromophores containing ligation by imidazole groups from histidine side chains. The synthesis, structural characterization, and spectroscopic study of a model chromophore containing apical (approximately 2.9 A) Cu-S bonding shows that S yields Cu(II) charge-transfer absorption of this type is very weak; bonding of this type is exhibited by plastocyanin (and probaby other type 1 blue copper proteins.) The x-ray structure of Cu(H2N(CH2)3NH(CH2)2NH(CH2)3NH2).2ClO4 suggests why this ligand exhibits such a large affinity for Cu(II); studies using rats show that this ligand causes more copper excretion than either d-penicillamine or triethylenetetraamine (trien).